Combining Nickel- and Zinc-Porphyrin Sites via Covalent Organic Frameworks for Electrochemical CO2 Reduction

Covalent organic frameworks (COFs) are ideal platforms to spatially control the integration of multiple molecular motifs throughout a single nanoporous framework. Despite this design flexibility, COFs are typically synthesized using only two monomers. One bears the functional motif for the envisioned application, while the other is used as an inert connecting building block. Integrating more than one functional motif extends the functionality of COFs immensely, which is particularly useful for multistep reactions such as electrochemical reduction of CO2. In this systematic study, we synthesized five Ni(II)- and Zn(II)-porphyrin-based COFs, including two pure component COFs (Ni100 and Zn100) and three mixed Ni/Zn-COFs (Ni75/Zn25, Ni50/Zn50, and Ni25/Zn75). Among these, the Ni50/Zn50-COF exhibited the highest catalytic performance for the electroreduction of CO2 to CO and formate at −0.6 V vs RHE, as was observed in an H-cell. The catalytic performance of the COF catalysts was further extended to a zero-gap membrane electrode assembly (MEA) operation where, utilizing Ni50/Zn50, CH4 was detected along with CO and formate at a high current density of 150 mA cm–2. In contrast, under these conditions predominantly H2 and CO were detected at Ni100 and Zn100 respectively, indicating a clear synergistic effect between the Ni- and Zn-porphyrin units.


INTRODUCTION
Electrochemical carbon dioxide reduction reaction (CO 2 RR) into C 1 products such as CO, formate, CH 4 , and methanol using renewable electrical energy is a promising route toward fossil-fuel-free feedstock. 1Since CO 2 has a high reduction reaction energy barrier, catalysts are required for its conversion. 2 Well-studied materials for this purpose are transition metal particles and surfaces, 3,4 as well as molecular catalysts. 5Continuous optimization of these catalysts aims to improve their stability, efficiency, and product selectivity, while new catalytic designs offer promise for new functionalities.However, controlling the product species through easily implementable chemical modifications of the catalyst remains a challenge.
Covalent organic frameworks (COFs) and metal−organic frameworks (MOFs) are emerging alternative catalysts that have shown a promising approach toward electroreduction of CO 2 with high tunability. 6Careful design of their originating monomers allows chemical and spatial control of their active sites, resulting in high degrees of control over product selectivity. 7,8At the same time, their nanoporous networks make the active sites highly accessible to CO 2 and product intermediates.COFs with strong polymer backbones, such as "locked" polyimines and polyimides, are receiving more attention due to their decent stability in aqueous electrolytes. 9,10Widely used catalytically active units in these COFs are porphyrins and phthalocyanines. 11These molecular motifs are highly tunable, since the type of metal-ion coordinated to the porphyrin or phthalocyanine ligand greatly impacts specific electro-or photocatalytic reactions. 12,13For example, in porphyrin-based COFs, the optimal metal-ion in the production of CO during electrochemical CO 2 RR is cobalt. 14,15On the other hand, Ni-and Zn-porphyrin-based COFs outperform Co-porphyrin-COFs in the photocatalytic hydrogen evolution reaction (HER). 16Much of the catalytic activity of these materials depends on the electronic configuration of the coordination complex between the metal-ion and the porphyrin ligands.This presents opportunities for the development of bifunctional (A-B) catalysts, where regulation of the ratio of A to B may provide control over the activity and selectivity during CO 2 electroreduction.
A bifunctional strategy has been employed in phthalocyanine MOF systems, where control of both the metal node of the MOF backbone and the metal ion coordinated to the phthalocyanine allows the formation of a true A-B bifunctional catalyst.For example, Zhong et al. 17 studied a system using either copper-or zinc-phthalocyanine linkers in MOFs structured by either copper-or zinc-bis(dihydroxy) nodes, totaling 4 different structures.Interestingly, the specific combination of a copper-phthalocyanine combined with a ZnO 4 backbone produced the largest Faradaic efficiency (FE) of 88% CO and a current density (j) of 4 mA cm −2 at −0.7 V vs a reversible hydrogen electrode (RHE).The conversions on the bifunctional catalyst were attributed to transfer of protons and electrons that were initially generated on the copperphthalocyanine sites during water electrolysis and were then transported to the zinc-active sites where CO 2 could be efficiently converted into CO.In a similar system using copperphthalocyanine linkers and CuO 4 nodes, Chen and coworkers 18 demonstrated that C 2 H 4 can be produced with FE = 50% and j = 7.3 mA cm −2 at −1.2 V vs RHE.They proposed that CO formation, followed by desorption from the copper-node of the backbone and finally diffusion toward the COcopper-phthalocyanine active site, enables C−C coupling.Thus, the incorporation of multiple catalytically active units within one framework may provide cooperativity in terms of the reaction mechanism.
In this work, we employed a systematic approach of mixing nickel-and zinc-porphyrin monomers at various ratios.Each monomer mixture was then utilized in the COF polymerization process, yielding multiple different COF catalysts for CO 2 RR.The COFs were synthesized under solvothermal conditions with tetraamino-functionalized metalloporphyrins with terephthaldehyde linkers.The molar ratios of nickel-to zinc-porphyrin units were varied: Ni 100 /Zn 0 , Ni 75 /Zn 25 , Ni 50 / Zn 50 , Ni 25 /Zn 75 , and Ni 0 /Zn 100 , totaling five different frameworks.After synthesis and characterization of the molecular and polymeric structure of all five COFs, their catalytic efficiency in CO 2 RR was investigated.The choice for a mixed

Metalloporphryin COF Structure Investigation.
Nickel-and zinc-containing 5,10,15,20-tetra(4-aminophenyl)porphyrin monomers (Ni-TAPP and Zn-TAPP) were synthesized via a 3-step route using commercial building blocks and subsequently analyzed (Scheme S1, Figure S1, and the Experimental Section).The polycondensation reactions of Ni-and/or Zn-TAPP with 2,5-dihydroxyterephthaldehyde (DHTA) under solvothermal conditions yielded COF structures with various ratios of nickel-and zinc porphyrins (Figure 1a).FT-IR spectroscopy, UV−vis spectroscopy, and TGA analysis (Figures S2−S5) were utilized to confirm the formation of these COF structures.The thermal properties of all COFs are similar.The mid-IR and UV−vis spectra of the COFs containing both Ni-and Zn-porphyrins appear to be intermediate between those of the two pure component COFs and follow a clear trend depending on the Ni:Zn ratio.
The nitrogen adsorption isotherms are shown in Figure 1b, and Figure S6.Although Ni/Zn-porphyrin COFs have different porosities depending on the ratio between originating Ni-and Zn-porphyrin monomers, their nitrogen isotherm curve shapes remain similar, following typical curves representative of micro-and small mesoporosity.Interestingly, the Ni 50 /Zn 50 COF appears to have the largest absolute microand mesopore volume.To expand on this, pore size distributions (PSDs) were calculated (Figure 1c).As seen here and Figures S7 and S8, all COFs contain a significant micropore volume (0.17−0.25 cm 3 •g −1 ) and small mesopore volume (0.06−0.13 cm 3 •g −1 ).Finally, surface areas based on BET theory using the BETSI program were calculated (Figures S9−S13), 19 as well as the Gurvich total pore volume for all COFs, and these values are depicted in the diagram in Figure 1d as a function of the ratio of Ni-and Zn-porphyrin units.An apparent optimum for both properties is seen in the Ni 50 /Zn 50 COF, having a BET surface area of 1180 m 2 •g −1 and a total pore volume of 0.67 cm 3 •g −1 .
X-ray photoelectron spectroscopy (XPS) was utilized to investigate the surface chemistry of the Ni/Zn-porphyrin COF powders (Figure S14) and atomic percentages of Ni and Zn based on these spectra were calculated (Table S1).The measured Ni:Zn ratios resemble the expected values based on the intended incorporation of Ni-and Zn-porphyrin units during COF synthesis, with slight deviations: 65:35, 40:60, and 29:71 for the Ni 75 /Zn 25 , Ni 50 /Zn 50 , Ni 25 /Zn 75 COFs, respectively.The polymer network was further investigated by powder X-ray diffraction (PXRD, Figure S15).Short range crystallographic order was detected for all COFs, and the diffractograms indicate a minimal distance of either 2.45 nm (in-plane) or 0.4 nm (interlayer) between neighboring metalloporphyrin sites.We observed correlations between this short-range order and the significant fraction of small mesopores (Figure 1c) identified through N 2 sorption measurements.On the other hand, the micropore volume likely originates from a combination of interlayer porosity and amorphous segments throughout the polymer network.
The sheet-like character of the COFs was further inspected using high-resolution transmission electron microscopy (HR-TEM).The morphology of all COFs revealed interconnected/ aggregated sheets with sizes in the range of 20−40 nm (Figure S16), of which a representative image (Ni 50 /Zn 50 ) is shown in Figure 2a.The expected square-geometry of the repeating unit of ∼2 nm is visible in these images.The typical square lattice that was observed for all COF structures, was subsequently related to the proposed molecular structure of the porphyrin-COFs (Figure S17).The distance of repeating units based on PXRD was also indicated here, showing a clear correlation between our PXRD and HR-TEM findings.Closer inspection of the sheet-like structure of the Ni 50 /Zn 50 COF was performed using scanning transmission electron microscopy (STEM) with a high-angle annular dark field (HAADF) detector, in combination with energy-dispersive X-ray (EDX) spectroscopy (Figure 2b,c).Focusing on the elements Ni and Zn allowed us to obtain a 2D projection of the spatial distribution of these elements throughout the COF structure.Although no actual atomic resolution can be reached (partly due to COF degradation under the beam after a prolonged time), the STEM-EDX map does indicate a relatively homogeneous distribution of Ni-and Zn-porphyrin units with no significant preference to form microdomains of either of the two metallic elements.

Electrochemical CO 2 Reduction Performance.
The electrocatalytic properties of the synthesized COFs were studied using linear sweep voltammetry (LSV, Figure 3a, and Figure S18).A two-compartment electrochemical H-cell setup that included a three-electrode system, including a silver chloride (Ag/AgCl) reference electrode, glassy carbon working electrode (GCE), and a platinum counter electrode, was used for these experiments.To gain insight into the catalytic activity and selectivity of the synthesized catalysts, chronoamperometry was performed at several potentials ranging from −0.4 to −0.8 V vs RHE in CO 2 -saturated 0.1 M KHCO 3 (Figure S19).In addition, the reduced gaseous and liquid products were periodically sampled from the cathodic chamber headspace and the electrolyte, respectively, and analyzed by gas chromatography (GC), high-performance liquid chromatography (HPLC), and NMR (Figures S20 and S21).The optimal overpotential (determined from the LSV curve onset) for CO 2 RR on Ni 0 /Zn 100 and Ni 75 /Zn 25 was found to be −0.7 V vs RHE, while a more anodic potential of −0.6 V vs RHE was observed in the cases of Ni 100 /Zn 0 , Ni 25 /Zn 75 and Ni 50 /Zn 50 (Figure 3, Table S2).The largest increase in current density coupled with a noticeable positive shift to the lowest onset potential (approximately −0.6 V vs RHE) was observed in the case of the Ni 50 /Zn 50 COF.Aside from H 2 , CO was the sole reduction product from Ni 0 /Zn 100 , Ni 100 /Zn 0 , and Ni 75 / Zn 25 COFs.In contrast, formate was detected, in addition to CO and H 2 , in the cases of Ni 25 /Zn 75 and Ni 50 /Zn 50 .Among the five modified electrodes, Ni 50 /Zn 50 exhibited the highest CO 2 RR FE of 79% (FE CO = 69% and FE formate = 10%), followed by Ni 0 /Zn 100 (FE CO = 46%), Ni 25 /Zn 75 (FE CO = 36% and FE formate = 8%), Ni 75 /Zn 25 (FE CO = 27%), and Ni 100 /Zn 0 (FE CO = 4.5%) at −0.6 V vs RHE.Turnover number (TON) and turnover frequency (TOF) values are calculated for the CO 2 -converting catalysts (Table S3), yielding a TOF of ∼3000 h −1 for the best performing catalyst Ni 50 /Zn 50 , highlighting that these COFs have a high catalytic activity.
Next, for a better understanding of the reaction kinetics of the synthesized catalysts, Tafel plots were generated within the overvoltage window ranging from 0.4 to 0.8 V (Figure S22).The measured values were 212, 184, 92, and 385 mV/dec for Ni 75 /Zn 25 , Ni 50 /Zn 50 , Ni 25 /Zn 70 , and Ni 0 /Zn 100 , respectively.The smallest Tafel slope was observed in the case of bimetallic COFs when the two metal centers are combined, which is possibly linked to facilitated electron transfer through their synergistic effect, in contrast to the monometallic Ni 0 /Zn 100 .
Although H-cells are useful for studying the catalysts' behavior, their performance is limited by low solubility of CO 2 in an aqueous solution, competing HER, and low current densities. 20,21Maximizing the interaction between the electrolyte and the gas pathway via the catalytic scaffold allows flow cells to overcome mass transport limitations and suppresses the HER. 22Therefore, the catalytic activity of the catalysts was investigated using a MEA cell through stepwise constant current densities ranging from 25 to 150 mA cm −2 (Figure 4a and Figure S23).A higher content of nickel (Ni 100 to Ni 50 ) seems to be beneficial when high current densities (100 and 150 mA cm −2 ) are applied, since these catalysts show both a stable signal, as well as relatively low cell voltages.In contrast, the cell voltages measured for Ni 25 /Zn 75 and Ni 0 /Zn 100 COFs are rather unstable at these higher current densities, which is possibly an effect of the smaller surface areas and pore volume (reduced CO 2 and product mass transfer) or catalyst conductivity.
The selectivity obtained for the pure component catalysts in the MEA cells (especially at 150 mA cm −2 ) reflect comparable results to the ones obtained in the H-cell setup: Ni 100 /Zn 0 generating predominantly H 2 and Ni 0 /Zn 100 a combination of CO and H 2 (Figure 4b and 4f).Generally, the COF catalysts in the MEA setup produce more CO than in the H-cell.Of note should be that in both setups, the catalyst matrices lack the presence of conductive agents (e.g., carbon nanotubes), which would increase the electrocatalytic performance even further as we have seen in previous works. 23Both pure component catalysts produce trace amounts of formate in MEA cells, of which the production is highest at 100 mA cm −2 .In contrast, methane is observed as a unique product for the Ni 50 /Zn 50 and Ni 25 /Zn 75 catalysts (Figure 4d,e), where a deviation from the linear average of the pure component catalysts is evident in the relatively large production of formate and CH 4 , especially in the case of Ni 50 /Zn 50 .
The selectivity of this catalyst is FE formate = 40% and FE CH4 = 11% at 100 mA cm −2 , and FE formate = 43% and FE CH4 = 14% at 150 mA cm −2 (Table S3).Larger amounts of formate and CH 4 are obtained at higher current densities, with a simultaneous decrease in CO production.The observed production of CH 4 in the MEA cell particularly is an effect of (next to catalyst activity) the cell design, improved mass transport, higher current density, and the electrolyte.Experimentally testing a wider range of electrolytes would further elucidate electrolyte effects on product formation. 24Finally, Ni 50 /Zn 50 and Ni 0 / Zn 100 catalysts were most effective in suppressing HER, both having a FE total of 88% for CO 2 RR products at 100 mA cm −2 .
The partial current densities for the reduced products differ significantly for the various Ni−Zn compositions (Figure S24).Considering the best-performing catalyst (Ni 50 /Zn 50 ), the partial current density for CH 4 and formate increased at higher current density.Generally, the CO partial current density peaks decreased when cell voltages are lower than −3.0 V.In contrast, the partial current density for H 2 production (j H2 ) grows monotonically when the cell potential is more negative.Thus, at more negative cell voltages, the HER becomes dominant.
Scanning electron microscopy (SEM) was utilized to study morphological changes to the polymer particles at the surface (Figure S25).The COF particles are clearly visible on the GDE surface, both before and after catalysis.The effect of the electroreduction on these particles is noticeable in the form of unidentified matter partly covering and interpenetrating the COF particles.XPS was also used to assess chemical changes within the COF after catalytic reactions (Figure S26).A clear difference in the low binding energy region was observed, where peaks at 294.4−294.7 eV were only visible after catalysis.These peaks are characteristic for potassium K 2p, which belongs to salt precipitation. 25On the other hand, the spectrum for nitrogen, deconvoluted into peaks at 397.5− 397.8 eV and 398.7−399.3eV, is similar for the electrodes before and after catalysis, suggesting (at least partly) retention of stability of both the porphyrin-ring, as well as imine polymer backbone.Also, the signals for Ni 2p and Zn 2p are present before and after catalysis and show no clear changes.Lastly, FT-IR spectra of the electrode surfaces showed a noticeable difference with peaks at 1618 and 1390 cm −1 , indicating characteristic vibrations of formate species, only visible after catalysis (Figure S27).Thus, while the COF particles remain stable throughout the electrocatalytic experiments presented here, optimization of their surface chemistry would be needed to ensure long-term stability and functionality.Alternatively, chemically and thermally more robust polymer backbones such as polyimides have been proven to withstand similar electrochemical environments for a large amount of cycles. 26 potential approach regarding catalyst stability would be to condense the amino-functionalized porphyrin monomers used in the current research with dianhydrides to yield similar polyimides.
To further shed light on the possible catalytic mechanisms using synthesized Ni/Zn COF catalysts, a 1:1 (w/w) physical mixture of Ni 100 /Zn 0 -and Ni 0 /Zn 100 -COFs (named Ni 50 + Zn 50 ) was fabricated by mixing 3.5 mg of each of these COFs in 4 mL of DMF through 40 min sonication.A gas diffusion electrode (GDE) based on this mixture was prepared via the same method as the other COF catalysts (see Experimental Section) and was tested as a control experiment using the same experimental MEA setup.As shown in Figure 5a, Ni 50 + Zn 50 exhibited more negative voltages than Ni 50 /Zn 50 .Ni 50 + Zn 50 produces higher quantities of H 2 and more comparable quantities of CO at all current densities than Ni 50 /Zn 50 (Figure 5b,c).Interestingly, Ni 50 + Zn 50 was also able to produce formate and even CH 4 (FE formate = 18% and FE CH4 = 2.2% at 100 mA cm −2 ), albeit at much lower Faradaic efficiencies than Ni 50 /Zn 50 .Apart from the slightly more pronounced formate production and the trace methane quantities, the catalytic activity of Ni 50 + Zn 50 resembles the linear average of the Ni 100 /Zn 0 and Ni 0 /Zn 100 catalysts quite well.

Structural Synergy in Mixed Nickel-and Zinc-
Porphyrin-Based COFs.Utilizing multiple different metalloporphyrin monomers with the same polymerizable functional groups in a single COF synthesis has rarely been explored 14 and not yet in a systematic manner as has been shown in this research.The chemical differences (Figures S2−S5 and S9) between the five studied frameworks were largely attributed to the implemented ratio of Ni-to Zn-porphyrin units.However, the structural differences in terms of porosity were remarkable, with the Ni 50 /Zn 50 COF having the highest porosity overall at an apparent optimum regarding the Ni-TAPP: Zn-TAPP ratio (Figure 1d).Regardless of the exact cause for this porosity difference (e.g., monomer geometry and reactivity, interplane and out-of-plane defects, and sheet stacking), here we discovered a yet unexplored design parameter to optimize the COF structure.
Additionally, STEM/EDX mapping did not indicate the presence of microdomains of the two elements but rather a relatively homogeneous spatial distribution of Ni-and Znporphyrin units in the mixed metalloporphyrin COFs (Figure 2).As such, the two different active sites are in close (nanometer) proximity with each other, which proves to be important in discussing the plausible mechanisms of reaction product formation during the CO 2 RR experiments.

Catalytic Synergy in Nickel-and Zinc-Porphyrin-Based COFs.
As mentioned in the Introduction, while cobalt or zinc metal centers would primarily yield CO as the major product, 14,17 introducing nickel (with its unique catalytic activity) could enable enhanced proton and electron transfer.This is evidenced by the Ni 50 /Zn 50 COF catalyst, which successfully produced CO, formate, and CH 4 during CO 2 RR in the MEA cell.Knowing that single-site nickel and zinc catalysts are generally known for producing H 2 and CO, respectively, 27−29 which is also aligned with the selectivity of the pure component catalysts in this work, detecting CH 4 with transfer of eight electrons and protons is an interesting achievement.We propose three potential pathways of CH 4 production, beginning with CO as the initial intermediate in the CO 2 reduction reaction (Figure S28).These pathways assume CO production occurs at zinc sites, since the pure component Ni 0 /Zn 100 catalyst mainly produces CO, whereas the Ni 100 /Zn 0 catalyst predominantly forms H 2 .Additionally, all pathways consider the retention of framework structural stability under the experimental CO 2 RR conditions, as has been indicated by XPS analysis (Figure S26).Stepwise desorption of CO from zinc, followed by adsorption onto nickel, where it could be converted into CH 4 is a plausible pathway and similar to the results of other works on stepwise CO 2 conversion. 18,30On the other hand, a likely mechanism could be hydride transfer from Ni-sites (Ni−H complexes being an intermediate in the water splitting reaction) to neighboring Zn−CO sites.Of note should also be the detection of CH 4 when using a physical mixture of Ni 100 /Zn 0 -and Ni 0 /Zn 100 -COFs, indicating that the observed synergistic effect is likely due to interlayer interaction rather than within a single plane.
To gain additional insights into the mechanism of CO 2 reduction, DFT calculations were performed to obtain binding energies between CO and Ni(II)-and Zn(II)TPP complexes (Table S5, Figure S29).Ni(II)TPP (high spin state, i.e., its ground state) and Zn(II)TPP show a binding enthalpy of −33.8 kJ/mol, and −29.1 kJ/mol, respectively, suggesting that CO binds more strongly with Ni(II)-TPP than with Zn(II)-TPP.These results add some weight to the earlier proposed mechanism of CO formation on Zn, followed by desorption/ adsorption on Ni(II) for further conversion.At the same time, it might still be true that other Ni(II)-centers also participate in the mentioned hydride transfer, possibly not only to Zn(II)-CO complexes, but, taken into account these calculations, also to Ni(II)-CO complexes.However, it is important to note that applying an electrochemical potential, which includes changes in the oxidation state of metalloporphyrins, can significantly affect their binding affinity with CO. 31,32 Therefore, further DFT studies on charged metalloporphyrins, as well as in situ characterization (e.g., utilizing a spectrochemical H-cell), should be conducted to deepen our understanding.

CONCLUSION
A systematic synthetic strategy was developed by combining Ni-and Zn-porphyrin monomers to yield bifunctional COFs (Ni 75 /Zn 25 , Ni 50 /Zn 50 , and Ni 25 /Zn 75 ), which were compared to the pure component Ni-and Zn-porphyrin COFs (Ni 100 and Zn 100 ).Structural synergy was discovered as the Ni 50 /Zn 50 COF exhibited the highest micro-and mesoporosity.Additionally, synergy in CO 2 RR catalytic activity between the two metal centers was apparent through the production of the relative largest amount of CH 4 using the Ni 50 /Zn 50 catalyst, compared to mostly H 2 and CO at the Ni 100 and Zn 100 catalysts, respectively.As such, this material platform allowed tunable product selectivity through simple adjustment of the Ni-and Zn-porphyrin monomeric ratio.Considering the wide range of metalloporphyrins available, we anticipate that this strategy will expand the library of bi-or even higher orderfunctional COF catalysts considerably and that such catalysts are then utilized to tackle complex multistep reactions as is shown here.
Analysis Techniques.NMR spectra were recorded at 298 K (unless stated otherwise) on an Agilent-400 MR DD2 spectrometer (400 MHz). 1 H NMR chemical shifts (δ) are given in parts per million (ppm) and were referenced to tetramethylsilane (0.00 ppm).Coupling constants are reported as J values in hertz (Hz).Data for 1 H NMR spectra are reported as follows: chemical shift (multiplicity, coupling constant, integration).Multiplicities are abbreviated as s (singlet) and d (doublet).FT-IR spectra were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer with a universal ATR accessory over a range of 4000−650 cm −1 .TGA analyses were performed from 30 to 860 °C under a nitrogen atmosphere at a heating rate of 10 °C•min −1 using a PerkinElmer TGA 4000.Prior to the measurement, the samples were degassed at 130 °C for 1 h under a nitrogen atmosphere.Liquid UV−vis spectra were recorded at 298 K on a PerkinElmer Lambda 35 UV−vis spectrometer (quartz cuvette) at a concentration of 5 μM in DMF.Prior to the measurements, the COF-DMF suspensions were sonicated for 30 min at room temperature.Nitrogen isotherms were measured on the NOVAtouch gas sorption analyzer from Quantachrome Instruments with high purity N2 (99.99%) at 77 K. Prior to the sorption measurements, all samples were degassed at 130 °C under vacuum for 16 h.The Quantachrome VersaWin software package was used for calculations of pore size distributions by fitting the nitrogen adsorption isotherms to the quenched solid density functional theory (QSDFT) carbon model (using slit/cylindrical/spherical pores).No smoothing factor was applied for the PSD calculation.X-ray photoemission spectroscopy (XPS) measurements were performed using a monochromatic Al Kα excitation source with a Thermo Scientific K-Alpha spectrometer.The spectrometer was calibrated using the C 1s adventitious carbon with a binding energy of 284.8 eV.The base pressure at the analysis chamber was about 2 × 10 −9 mbar.The spectra were recorded using a spot size of 400 μm at a pass energy of 50 eV and a step size of 0.1 eV.PXRD patterns were measured on a Rigaku MiniFlex 600 powder diffractometer using a Cu Kα source (λ = 1.5418Å) over the 2θ range of 2−40°with a scan rate of 1°•min −1 .For high-resolution transmission electron microscopy analysis, a FEI cubed Cs corrected Titan was used.HREM lattice images are collected on a Thermo Scientific Ceta 16M.A low intensity on the camera was used to avoid beam damage.In scanning mode (STEM) ADF (annular dark field) images are collected.In this mode, a subnanometer beam is scanned on the electron transparent sample and for each beam position the diffracted electrons are collected on a ring shape detector.On heavy/thicker parts of the sample, more diffracted electrons are collected, showing up bright in the image.Elemental mapping in STEM mode was done, using the super-X in the ChemiSTEM configuration.The EDX spectrum is collected for each beam position in a STEM image.The accelerating voltage during STEM and TEM was 300 kV.COF framework degradation was observed after prolonged exposure under this beam.Therefore, images and elemental maps were collected in the first few 1−3 min, before the onset of degradation.For TEM sample preparation, the COF powder was crushed in a mortar first without and then under some ethanol.The dispersion was ultrasonically shaken for 5 min.Using a pipet, the dispersion was drop casted onto a C foil supported with a Cu grid (holey Quantifoil TEM grid).After drying, the grid was ready for TEM inspection.Scanning electron microscopy (SEM) images were recorded with a JEOL JSM-840 SEM: materials were deposited onto a sticky carbon surface on a flat sample holder, vacuum-degassed, and sputtered with gold at a thickness of 15 nm.The data concerning the characterization of the materials described in this work can be accessed and used by others for further studies at 4TU.ResearchData. 33ynthesis of H 2 TNPP (1a, Scheme S1).A solution of 4nitrobenzaldehyde (18.9 g, 125 mmol, 4.0 equiv) in propionic acid (500 mL) and acetic anhydride (23.6 mL) was heated to 150 °C.Then, pyrrole (8.7 mL, 125 mmol, 4.0 equiv) was added dropwise and the resulting black mixture was refluxed for 30 min at 150 °C.After cooling to rt, the precipitate was successively filtered off, washed with water (200 mL), and dried under high vacuum.Pyridine (200 mL) was added to the black solid, and the suspension was refluxed for 30 min.After cooling to rt, the mixture was stored at 4 °C for 16−20 h.The resulting precipitate was successively filtered off, washed with acetone (200 mL), and dried under high vacuum to afford TNPP (5.5 g, 22%) as a dark purple solid.Due to the poor solubility of H 2 TNPP, it was immediately used in the next step without characterization.
Synthesis of H 2 TAPP (1b, Scheme S1).H 2 TNPP (5.5 g, 6.9 mmol, 1.0 equiv) was suspended in 37% hydrochloric acid (250 mL), and the resulting mixture was stirred at 20 °C for 30 min.Then, tin(II) chloride dihydrate (23.3 g, 103 mmol, 15 equiv) was added and the reaction mixture was stirred at 80 °C for 30 min.Upon cooling, the mixture was cooled further to 0 °C and carefully neutralized by the addition of ammonium hydroxide (300 mL).The resulting precipitate was filtered off, and air-dried.The black solid was suspended in THF (50 mL) and stirred at 20 °C for 15 min and then it was filtered.Heptane (200 mL) was added to the filtrate to precipitate the product.Most THF was evaporated under reduced pressure and the resulting suspension was centrifuged.The supernatant was removed, the precipitate was washed with pentane (100 mL) and dried under high vacuum to afford H 2 TAPP (1.5 g, 33%) as a purple solid. 1 H NMR (400 MHz, (CD 3 ) 2 SO) δ (ppm): −2.77 (s, 2H), 5.53 (s, 8H), 6.97 (d, J = 8.3 Hz, 8H), 7.81 (d, J = 8.3 Hz, 8H), 8.84 (s, 8H).Spectral data were in agreement with literature values. 3ynthesis of Ni-TAPP (2, Scheme S1).A solution of H 2 TAPP (530 mg, 0.79 mmol, 1.0 equiv) and nickel(II) acetate tetrahydrate (2.0 g, 7.9 mmol, 10 equiv) in DMF (100 mL) was heated at 100 °C for 16−20 h.After cooling to rt, water (300 mL) was added.The resulting precipitate was filtered off, and air-dried.The residue was dissolved in THF and filtered through a plug of silica.The purified material was dissolved in minimal THF and precipitated by the addition of hexane.Most THF was evaporated under reduced pressure and the resulting suspension was centrifuged.The supernatant was removed, the precipitate was washed with hexane (100 mL) and dried under high vacuum to afford Ni-TAPP (298 mg, 52%) as a red solid. 1 H NMR (400 MHz, (CD 3 ) 2 SO) δ (ppm): 5.46 (s, 8H), 6.88 (d, J = 8.4 Hz, 8H), 7.60 (d, J = 8.3 Hz, 8H), 8.73 (s, 8H).Spectral data were in agreement with literature values. 3ynthesis of Zn-TAPP (3, Scheme S1).A solution of H 2 TAPP (506 mg, 0.75 mmol, 1.0 equiv) and zinc acetate dihydrate (1.6 g, 7.5 mmol, 10 equiv) in chloroform/methanol (100 mL, 1:1, v/v) was refluxed for 16−20 h.After cooling to rt, triethylamine (2 mL) was added and the mixture was evaporated to dryness.The residue was dissolved in THF and filtered through a plug of silica.The purified material was dissolved in minimal THF and precipitated by the addition of hexane.Most THF was evaporated under reduced pressure and the resulting suspension was centrifuged.The supernatant was removed, the precipitate was washed with hexane (100 mL) and dried under high vacuum to afford ZnTAPP (347 mg, 63%) as a green solid. 1 H NMR (400 MHz, (CD 3 ) 2 SO) δ (ppm): 5.41 (s, 8H), 6.93 (d, J = 8.3 Hz, 8H), 7.77 (d, J = 8.3 Hz, 8H), 8.80 (s, 8H).Spectral data were in agreement with literature values. 3ynthesis of Ni-/Zn-porphyrin COFs.Ni(II)-5,10,15,20tetrakis(4-aminophenyl)porphyrin (Ni-TAPP) and Zn(II)-5,10,15,20-tetrakis(4-aminophenyl)porphyrin (Zn-TAPP) compounds were synthesized and analyzed, as detailed in the Supporting Information.X% Ni-TAPP and (100 − X)% Zn-TAPP, combined, totaling 0.04 mmol, were added to a 20 mL prescorched borosilicate ampule.6 mL of the solvent mixture (1:1 (v/v) mixture of orthodichlorobenzene and 1-butanol) was added to the ampule, after which it was sonicated for 1 min.Then, 0.08 mmol of 2,5-dihydroxyterephthaldehyde was separately suspended in 1 mL of acetic acid (6 M in water) and 2 mL of solvent mixture and subsequently dropwise added to the ampule.The mixture in the ampule was briefly homogenized and subjected to three freeze−pump−thaw cycles.Lastly, the ampule was flame-sealed and left in an oven at 120 °C for 3 days.The workup of the COFs included washing with THF (∼6 × 10 mL) until the washing solution was clear of color and the COFs were subsequently washed with acetone (3 × 10 mL).Thereafter, the powders were dried at 60 °C in a vacuum oven for 16 h.The yields of the COFs were: Ni 100 /Zn 0 (34.0 mg, 86%), Ni 75 /Zn 25 ( Preparation of Deposited COF Complexes onto Electrodes.The mixture of each COF compound (7 mg) in DMF (4 mL) with 5 wt % Nafion was sonicated for 40 min to obtain a well-mixed suspension.Then, the mixture was stirred at room temperature overnight and subsequently drop-casted onto a gas diffusion electrode (GDE, Sigracet 38 BC, 5% PTFE applied nonwoven carbon paper with a microporous layer; 2.5 cm × 2.5 cm) for the membrane electrode assembly (MEA) study.For the H-cell setup, 10 μL of the prepared suspension was drop-casted on the preprepared surface (d = 3.0 mm) of a standard glassy carbon electrode and let to dry for 24 h.All potentials were reported versus the Ag/AgCl reference electrode.Potentials were changed from Ag/AgCl (3 M KCl) to the reversible hydrogen electrode (RHE, E RHE = E Ag/AgCl + 0.059 × pH + 0.210).
Characterizations during Electroreduction.The reduced products observed in the cathodic compartment were periodically collected from the reaction headspace and tested by gas chromatography (GC).The concentration of gaseous products (CO, CH 4 , H 2 ) was obtained from GC, and the average of 4 injections was used to calculate their Faradaic efficiencies.The gas product from CO 2 electroreduction was analyzed using a chromatograph (InterScience PerkinElmer Clarus 680) coupled with two thermal conductivity detectors (TCD) and a flame ionization detector (FID), while the liquid product was analyzed using HPLC (Infinity 1260 II LC, Agilent Technologies, Hi-Plex H column (at 50 °C) with VWD (at 210 and 280 nm) and RID (at 40 °C)) (Figures S12 and  S13). 1 H NMR was measured using a Bruker 400 MHz setup and the data were processed in MestreNova.The chemical shifts (δ) are reported in ppm.
H-Cell and Membrane Electrode Assembly (MEA) Experiments.To evaluate the electroactivity of the synthesized COF complexes, the electrochemical reduction of CO 2 was first studied with an H-cell using the linear sweep voltammetry (LSV) technique.The two-compartment H-cell comprised a three-electrode configuration, including the immobilized COF catalysts on a glassy carbon working electrode (GCE), a silver/silver chloride (Ag/AgCl) reference electrode, and a platinum (Pt) counter electrode in a CO 2 -saturated 0.1 M KHCO 3 aqueous solution.Gas-phase products were collected from the reaction headspace and measured using gas chromatography (GC).For experiments with higher current densities, a membrane electrode assembly (MEA) electrolyzer consisting of an anode chamber (Ni-foam anode, Recemat BV) with a liquid phase anolyte (0.5 M KOH) and a cathode chamber (COF on GDE) with a gas phase inlet was employed (schematic shown at Figure S23).The membrane that separates these chambers is a Sustainion anionexchange membrane (X37-50 grade RT).In this design, gaseous CO 2 is delivered directly (at 40 mL min −1 , STP) to the active materials through an inlet located at the back side of the GDE.
Faradaic Efficiency Calculation (for Both H-Cell and MEA).Gas phase mole fractions were determined using GC injections periodically (and averaged over 4 times) every 5 min during electrolysis (after stabilization periods).Liquid mole fractions were determined using NMR analysis.To estimate the Faradaic efficiency of gaseous products, the mole fractions of CO and H 2 were calculated from GC injections.Under constant pressure and temperature (ideal gas law), the volume fraction of the gas products (from GC) equals their corresponding mole fraction.The amount of water vapor exiting the reactor was measured using a humidity sensor and found to be 78% relative humidity, which corresponds to a mole fraction of water of 2.3% (x H2O = 0.023).Since the sum of mole fractions is equal to 1, the mole fraction of CO 2 exiting was calculated as eq 1.
After calculating the mole fractions of all gaseous products, the volumetric flow rate at the reactor outlet (sccm units) was measured with a mass flow meter and used to calculate the moles of each product.
e.g., n CO is mol/s of CO produced, n e is the number of electrons involved in CO 2 RR (2 for CO), F is 96485 C mol −1 , and I is the applied current (in amperes).T cal and P cal refer to the calibration T and P of the mass flow meters.
Computational Study.DFT calculations were done using version 5.0.4 of the ORCA package 34 and the PBE0 functional 35 with Grimme's D3 dispersion correction with Becke-Johnson damping 36 using the def2 family of basis sets. 37The solvation model based on density (SMD) 38 was applied to simulate implicit water around the molecules.The calculations were done on the Delft Blue super computer. 39First, the structures were optimized at def2-SVP level of theory.Then, single-point calculations and frequency calculations were done at def2-SVP level with the def2-TZVPP basis set on the metal atoms to obtain the energies and check that the structure is at an energetic minimum.Bond enthalpies between CO and MTPP were obtained by comparing the enthalpies of the optimized MTPP-CO, MTPP and CO structures.Calculations for NiTPP and NiTPP-CO complexes were performed in both singlet and triplet states to account for low spin (LS) or high spin (HS) ground states, while ZnTPP and ZnTPP-CO complexes as well as CO were kept in singlet state.

Figure 1 .
Figure 1.(a) Chemical structure of Ni/Zn-porphyrin COFs and their monomers.(b) Nitrogen adsorption isotherms of Ni/Zn-porphyrin COFs measured at 77 K. (c) Pore size distributions of Ni/Zn-porphyrin COFs, calculated from experimental N 2 adsorption isotherm branches and based on a QSDFT carbon model with slit/cylindrical pore geometries.(d) BET surface areas and Gurvich total pore volumes of Ni/Zn-porphyrin COFs, calculated from experimental N 2 adsorption isotherms.